The present invention is related to Japanese patent application No. Hei. 2000-10351, filed Jan. 17, 2000; the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a vehicular head up display, and more particularly, to a vehicular head up display that adjusts luminescence according to outside conditions.
2. Background of the Invention
A conventional vehicular head up display of this type is disclosed in documents such as JP-B2-2861809 (U.S. Pat. No. 5,572,342). Here, the head up display includes a liquid crystal panel provided on the back-surface side of an instrument panel in the vehicle compartment, and a light source provided on the back side of the liquid crystal panel. A reflecting mirror reflects a display light, which is based on a light generated by the light source and is emitted from the liquid crystal panel. The reflected display light propagates to a front wind shield by way of an upper wall opening bored through the instrument panel. The front wind shield forms a virtual image in front of the driver to display information generated by the liquid crystal panel, so that the driver can visually recognize the display information.
The head up display employs a xenon lamp or a halogen lamp generally having an embedded single filament as a light source. However, such a light source has a problem that, if the luminance of an emitted light is changed by varying an applied voltage, the color of the light also changes as well. In other words, the color of light emitted by the light source varies when the brightness of the light is changed.
In order to solve the problem, a detailed study was conducted. A result of the study indicates that, when the weather is good during day time, the head up display is generally required to emit a light with a highest degree of luminance in order to allow the driver to visually recognize the virtual image with a high degree of reliability. At night, excessively high luminance of light emitted by the light source reversely results in too bright virtual image, which adversely becomes a disturbance to the driving operation of the driver. Thus, in order to let the driver visually recognize the virtual image information well, it is necessary to generate a light with luminance thereof adjusted to at least about 1/100 of luminance ratio without a recognized change in emitted-light color.
In the case of a light source having an embedded single filament as described above, in order to prevent the driver from recognizing a change in emitted-light color, the luminance of the emitted light cannot be changed to a magnitude smaller than about 1/10 of luminance ratio.
A relation between the luminance of an emitted light and an applied voltage for a xenon lamp was studied. As a result of the study, a characteristic represented by a graph shown in FIG. 5 was obtained. In addition, a relation between X chrominance and Y chrominance on a Munsell color table to an applied voltage for a xenon lamp was also studied. As a result of the study, characteristics represented by graphs b and c shown in FIG. 6 were obtained. To be more specific, the graph b represents a relation between X chrominance and an applied voltage whereas the graph c represents a relation between Y chrominance and an applied voltage. It should be noted that, as a sample of the xenon lamp, a C-2V xenon lamp made by TOSHIBA LIGHTING and TECHNOLOGY CORPORATION was used in the study. As for the measurement apparatus, a BM-5A Chrominance Measurement Instrument made by Topcon Corporation was used.
As is obvious from the graph shown in FIG. 5, the luminance of light emitted by the xenon lamp at applied voltages in a range lower than about 6.8 V is very small. Expressed in terms of a luminance ratio to a predetermined reference, the value is equal to or smaller than 0.1. For applied voltages in a range higher than about 6.8 V, on the other hand, the luminance increases substantially with the increases in applied voltage. As is obvious from the graphs b and c shown in FIG. 6, the X chrominance of light emitted by the xenon lamp at applied voltages in a range lower than about 6.8 V is far separated from the Y chrominance. For applied voltages in a range higher than about 6.8 V, on the other hand, the X chrominance of light emitted by the xenon lamp is close to the Y chrominance.
Thus, in order to prevent the driver from recognizing a change in chrominance of light emitted by the xenon lamp, it is necessary to use the luminance of a light at applied voltages in a range higher than about 6.8 V. In this range, however, the luminance of the emitted light cannot be changed to a magnitude smaller than about 1/10 of luminance ratio as described above and as evidenced by the graph shown in FIG. 5.
Note that, as a light source of the head up display, it is also possible to use a light source having two embedded filaments as is disclosed in JP-U-6-23156, JP-U-6-50238. However, the problem of the luminance of the emitted light cannot be solved by merely using the two-filament light source.
It is thus an object of the present invention to provide a vehicular head up display that employs a plurality of embedded filaments with luminance levels different from each other as a light source of a liquid crystal display unit and allows luminance to be varied over a wide range while properly suppressing changes in color of light emitted by the light source.
According to a first aspect of the present invention, there is provided a vehicular head up display comprising a light source provided on the back-surface side of an instrument panel located below a front wind shield inside a vehicle; a liquid crystal display unit provided on the back-surface side of the instrument panel in front of the light source and used for emitting a light originated from the light source as a display light representing information; and an optical element for passing the display light emitted by the liquid crystal display unit by way of an opening bored through the instrument panel as an image-formation light on the front wind shield, wherein the front wind shield reflects the image-formation light generated by the optical element and displays the information as a virtual image.
The head up display further has a brightness-sensing means for sensing brightness outside the vehicle; a setting means for setting a light-control quantity for the light source at a value corresponding to the brightness outside the vehicle; and control means to 246, and for controlling luminance of light emitted by the light source in accordance with the brightness outside the vehicle and the set light-control quantity.
The light source has a plurality of embedded filaments and having luminance levels different from each other. The control means controls the luminance of light emitted by the light source by driving a filament with low luminance selected among the filaments. A filament is selected in such a way that, the smaller the set light-control quantity, the lower the luminance of the selected filament.
As described above, the head up display employs a light source having a plurality of embedded filaments and having luminance levels different from each other. In addition, the control means control the luminance of light emitted by the light source by driving a filament that has low luminance and is selected among the filaments in such a way that, the smaller the set light-control quantity, the lower the luminance of the selected filament. Thus, the control means controls the luminance of light emitted by the light source by assigning one of the filaments to cover a portion of a range of changes in luminance of the light emitted by the light source. As a result, it is possible to let the driver recognize display information of the liquid crystal panel as a virtual image while suppressing shifts in color of the light emitted by the light source 10b to a level unrecognizable by the driver.
In a vehicular head up display provided by another aspect of the invention, a liquid crystal display unit receiving a light from a light source represents information as a display light, forming an image on a reflecting member in front of the driver seat in the vehicle to be recognized by the driver.
This head up display comprises a brightness-sensing means for detecting brightness outside the vehicle; and control means for controlling luminance of light emitted by the light source in accordance with the brightness outside the vehicle.
The light source has a plurality of embedded filaments having luminance levels different from each other. The control means control the luminance of light emitted by the light source by driving a filament with low luminance selected among the filaments. A filament is selected in such a way that, the smaller the amount of brightness, the lower the luminance of the selected filament.
In this way, the control means control the luminance of light emitted by the light source by driving a filament that has low luminance and is selected among the filaments in such a way that, the smaller the amount of the brightness, the lower the luminance of the selected filament. By controlling the luminance of light emitted by the light source in this way, the same effect as the head up display according to the first aspect can be obtained.
In addition, a vehicular head up display provided according to a third aspect of the invention comprises a light source provided on the back-surface side of an instrument panel located below a front wind shield inside a vehicle; a color liquid crystal display unit provided on the back-surface side of the instrument panel in front of the light source and used for emitting a light from the light source as a display light representing information; and an optical element for passing the display light emitted by the color liquid crystal display unit by way of an opening bored through the instrument panel as an image-formation light on the front wind shield, wherein the front wind shield reflects the image-formation light generated by the optical element and displays the information as a virtual image.
The head up display further has a brightness-sensing means for sensing brightness outside the vehicle; a setting means for setting a light-control quantity for the light source at a value corresponding to the brightness outside the vehicle; and control means, for controlling luminance of light emitted by the light source in accordance with the brightness outside the vehicle and the light-control quantity.
The light source has a plurality of embedded filaments having luminance levels different from each other. In the case of a small set light-control quantity, the control means control the luminance of light emitted by the light source by driving a filament with low luminance selected among the filaments. In the case of a large set light-control quantity, on the other hand, the control means control the luminance of light emitted by the light source by driving at least a filament with high luminance selected among the filaments.
As described above, the light source 10b has a plurality of embedded filaments having luminance levels different from each other wherein, in the case of a small set light-control quantity, the control means control the luminance of light emitted by the light source by driving a filament with low luminance selected among the filaments while, in the case of a large set light-control quantity, the control means control the luminance of light emitted by the light source by driving at least a filament with high luminance selected among the filaments. As a result, the same effect as the head up display according to the first aspect can be obtained.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. In the drawings: